1. Field of the Invention
The invention relates generally to electrical switching apparatus and, more particularly, to contact assemblies for electrical switching apparatus, such as circuit breakers. The invention also relates to movable contact arms for circuit breaker contact assemblies.
2. Background Information
Electrical switching apparatus, such as circuit breakers, are employed in diverse capacities in power distribution systems such as, for example, to provide protection for electrical equipment from electrical fault conditions (e.g., without limitation, current overloads; short circuits; abnormal level voltage conditions).
As shown in FIGS. 1 and 2, a circuit breaker 2 (FIG. 1) generally includes a housing 4 which encloses a line conductor 6, a load conductor 8 (FIG. 1), a fixed contact 10 and a movable contact 12, with the movable contact 12 being movable into and out of electrical contact with the fixed contact 10. This switches the contacts 10, 12 of the circuit breaker 2 between the OFF or open position shown in FIG. 1, and the ON or closed position (as best shown in FIG. 3), or between the ON or closed position and a tripped or tripped off position (not shown). In the example shown, the fixed contact 10 is electrically connected to the line conductor 6 and the movable contact 12 is electrically connected to the load conductor 8 through a movable contact arm 16 by a suitable conductor, such as a flexible conductor (not shown). The circuit breaker 2 further includes an operating mechanism 14 (FIG. 1) having the movable contact arm 16 upon which the movable contact 12 is disposed. The movable contact arm 16 and movable contact 12 disposed thereon move past and/or through an arc chute 18 which includes a plurality of arc plates 20 structured to attract and dissipate the resultant arc which is formed when the movable contact 12 initially separates from the fixed contact 10 in response to the trip condition.
The movable contact arms of many known circuit breakers, such as movable contact arm 16 of circuit breaker 2 (FIG. 1) are made of solid copper or alloys of copper (e.g., silver bearing copper; a copper alloy with a relatively small percentage of silver), which are relatively good conductors of both electricity and heat, but which are not as strong as other materials. Hence, it is believed that relatively more copper than is necessary to handle the current (e.g., for thermal conductivity considerations) is typically employed in conventional movable contact arms 16 to handle the current and to provide the needed strength (e.g., rigidity). This undesirably adds weight, thus increasing the moment-of-inertia of the movable contact arm 16 and decreasing the performance of the circuit breaker 2. More specifically, the movement-of-inertia of the movable contact arm 16 significantly affects the angular opening velocity of the movable contact arm 16. It is known that the faster the movable contact arm 16 opening velocity is, the better the current-limiting capability of the circuit breaker 2. Therefore, it is desirable to maximize the opening velocity of the movable contact arm 16 in order to improve the short-circuit interruption performance of the circuit breaker 2. Previously, this has not been possible because material strength and thermal requirements have dictated the size and geometry of the movable contact arm 16.
For example, the movable contact arm 16 shown in FIGS. 1, 2, and 3 is a single-piece arm 16 made from copper, as previously noted. In order to achieve the desired strength, the length 22 (i.e., the distance between the pivot point of the arm 16 and the end carrying the movable contact 12) (FIGS. 1 and 2) of the movable contact arm 16 is required to be relatively short, and the width 24 (FIGS. 2 and 3) of the movable contact arm 16 must be relatively wide. Specifically, it is believed that the ratio of the width 24 to length 22 is about 1:7.3, or more. The width 24 (FIGS. 2 and 3) is also greater than desired with respect to the height 26 (FIG. 3) of the movable contact arm 16. Specifically, it is believed that the ratio of the width 24 to the height 26 is about 1:2, or more. The foregoing results in the weight and the movement-of-inertia of the movable contact arm 16 being greater than desired, and the aerodynamic efficiency of the movable contact arm 16 being less than desired, thus adversely affecting the angular opening velocity of the movable contact arm 16 and inhibiting the circuit interruption performance of the circuit breaker 2.
There is a need, therefore, to provide a movable contact arm 16 sized and shaped to optimize the angular opening velocity of the arm 16, while exhibiting sufficiently high strength and thermal conductivity, and low electrical resistivity.
It is also desirable to maximize the space or gap 28 (FIG. 1) between the movable and fixed contacts 10,12 in order to minimize the undesired continued flow of electrical current following the trip condition. Such current, commonly referred to as let-through current, must be minimized in order to protect electrical components from the harmful effects of over-current resulting from the trip condition.
There is, therefore, room for improvement in contact assemblies for electrical switching apparatus and in movable contact arms therefor.